Imaging system and method for optimizing an x-ray image

ABSTRACT

The invention relates to an imaging (X-ray) system for observing the motion of an object in the vascular system of a body volume ( 10 ). An X-ray apparatus ( 3 ) in this system generates two-dimensional projection images ( 4 ) of the body volume ( 10 ). In a module ( 5 ) the position of the tip of the object is determined from the projection images and this position is associated, in a further module ( 2 ), with a previously acquired three-dimensional representation ( 1 ) of the vascular system. The module ( 2 ) then calculates optimum imaging parameters which involve notably a planar projection of the tip of the object and a minimum projection window. These parameters are subsequently set on the X-ray apparatus ( 3 ) so as to serve as a basis for the next two-dimensional image ( 4 ).

The invention relates to a method of optimizing a two-dimensional imageof a body volume which contains an object, as well as to an imagingsystem which is arranged to carry out such a method.

Imaging methods that generate a two-dimensional image of a body volumeare used in various fields of application. The generating oftwo-dimensional (X-ray) images of a biological body volume will beconsidered hereinafter by way of example; an object such as, forexample, the tip of a catheter or a guide wire then moves in the bloodvessels within said body volume. The invention, however, is by no meansrestricted to such applications and can be used in all cases withsimilar circumstances.

During the movement of an object through the body of a patient theobject follows the course of the vessels; this often gives rise to achange of direction. An imaging system for generating a two-dimensionalprojection of the body volume containing the object, therefore, must becontinuously readjusted in order to ensure optimum imaging of the objectin the current position. In this respect “optimum” usually means aplanar projection of the object or the surrounding segment of thevascular system. Such readjustment is very time-consuming for themedical staff and leads to an additional radiation burden for thepatient during the readjustment.

From prior art it is known to generate and store three-dimensionalrepresentations of the vascular system of a given body volume.Representations of this kind can be acquired by means of various imagingmethods such as computer tomography (CT), magnetic resonance (MR),rotation angiography (RA) or three-dimensional ultrasound (3DUS).Moreover, from U.S. Pat. No. 6,317,621 B1 it is known to combine athree-dimensional representation of the vascular system with a currenttwo-dimensional projection image in such a manner that the currentposition of a catheter can be determined and associated with thethree-dimensional representation. To this end, a number of markers areprovided on the body of the patient; such markers are reproduced in thethree-dimensional data as well as in the current projection images sothat they can be associated with one another.

Considering the foregoing it was an object of the present invention toprovide an imaging system and a method for the operation thereof whichenable a comparatively simple optimization of the representation of abody volume with an object contained therein. Preferably, the radiationload should be minimized for the body volume.

This object is achieved by means of a method as disclosed in thecharacterizing part of claim 1 as well as by means of an imaging systemas disclosed in the characterizing part of claim 3. Advantageousembodiments are disclosed in the dependent claims.

The method in accordance with the invention for optimizing atwo-dimensional image of a (biological or non-biological) body volumecontaining an object is characterized in that

a) a three-dimensional representation of feasible locations of theobject within the body volume is acquired, feasible locations being, forexample, trajectories or channels in the body volume along which theobject can move,

b) the current position of the object is determined and associated withthe three-dimensional representation (this means that the data pointassociated with the current position of the object is identified fromamong the data constituting the three-dimensional representation),

c) imaging parameters are determined by means of the three-dimensionalrepresentation, which imaging parameters are optimum in respect of thecurrent position of the object, in conformity with a predeterminedoptimization criterion,

d) a two-dimensional image of the body volume is generated by means ofsaid optimum imaging parameters, which image need not necessarily coverthe entire body volume and may be limited to a part of interest.

The described method utilizes the data of a three-dimensionalrepresentation of all feasible locations as well as the current locationof the object so as to calculate automatically parameters for an optimumtwo-dimensional image and to generate a corresponding image. Thetwo-dimensional representation of the body volume can thus be optimizedfor many important applications, without it being necessary for a humanoperator to carry out adjustments or to acquire test images. Therefore,optimized images can be acquired in an automated fashion, that is,within a substantially shorter period of time and also with a smallerradiation load for the body volume.

The two-dimensional image optimized by means of the method may inprinciple be any kind of image whereby a two-dimensional representationis formed from a volume. For example, it may be a sectional image formedby means of an ultrasound apparatus. The two-dimensional image, however,may in particular be a projection of the body volume which is generatedby means of X-rays. This type of imaging is suitable particularly forthe observation of the motion of an object through a body volume,because the image thus arising contains information from the entirevolume so that the object is included in any case.

Knowledge of the current position of the object is required in order tocarry out the described method. This knowledge may originate inprinciple from any suitable source of information, for example, from aseparate imaging method, from a localization method utilizingelectromagnetic field measurements (“active localizer”) or, in specialapplications, also from the determination of the configuration in spaceof an instrument carrier projecting from the body volume. Preferably,the position of the object is determined from a first two-dimensionalimage which has been formed by means of the same method as the optimizedtwo-dimensional image, because only a single imaging system will berequired in that case.

The nature of the imaging parameters that are optimally determined bythe method is governed by the respective imaging method used. In thiscontext notably the following imaging parameters may be involved: thesectional plane of an image, a projection direction, the position(location, orientation) of a radiation source, the position of animaging radiation detector, the shape (including the size) of an imagingwindow, the position of radiation-attenuating diaphragm elements,variances in the radiation field across an irradiated surface, theradiation quality (for example, adjustable by means of filters), theradiation intensity, the electrical current and/or the electricalvoltage for operating a radiation source and/or the exposure time.

An important field of application of the method is the use of an imagingsystem in the field of medical diagnostics and therapy. The feasiblelocations of the object may then notably be blood vessels within abiological body volume, the optimum image parameters in that case beingdefined in such a manner that the local vascular segment in which theobject is situated at the relevant instant is projected in thetwo-dimensional image in an essentially planar fashion; this means thatit is projected from a direction perpendicular to the axis of thevascular segment onto a plane parallel to the axis of the vascularsegment. In the context of a medical application the object may notablybe a catheter, or the tip thereof, a guide wire or the like. Thethree-dimensional representation of the vascular system can be acquirednotably by means of CT, MR, RA and/or 3DUS.

The two-dimensional image of the body volume can be advantageouslydisplayed so as to be superposed on an image of the three-dimensionalrepresentation which has been acquired at least partly with the sameimaging parameters. For example, when the two-dimensional image is aprojection of the body volume, a projection with the same projectiongeometry can be calculated from the three-dimensional representation soas to be used for the superposition. The information contained in thethree-dimensional representation is thus additionally made available tothe user. It is very advantageous when the image calculated from thethree-dimensional representation reproduces an area which is larger thanthe two-dimensional image. The “live” two-dimensional image of thecurrent position of the object can thus be limited to a minimum sizewhile minimizing the radiation load, because the user can extractinformation for the orientation in the further vicinity of the objectfrom the superposed image derived from the three-dimensionalrepresentation.

The invention also relates to an imaging system for generating atwo-dimensional image of a body volume which contains an object, whichsystem comprises a data processing unit for image processing and controlwhich includes a memory which stores a three-dimensional representationof feasible locations of the object within the body volume. The dataprocessing unit is also arranged to determine imaging parameters whichhave been optimized in respect of the current position of the object inconformity with a given optimization criterion from thethree-dimensional representation stored in the memory. Furthermore, thedata processing unit is arranged to control the imaging system in such amanner that it generates a two-dimensional image with the previouslymentioned optimized imaging parameters.

An imaging system of this kind offers the advantage that it utilizes athree-dimensional representation of the body volume and acorrespondingly configured data processing unit for the automaticcalculation of optimum imaging parameters for the relevant position ofthe object so as to generate a corresponding two-dimensional image. Theuser of the imaging system, therefore, need not carry out theseoperations and the formation of test images, giving rise to a radiationload, can be dispensed with.

The imaging system is preferably an X-ray apparatus which comprises anX-ray source and a detector, both of which are attached to a movableC-shaped arm. X-ray apparatus of this kind are used notably in themedical field where the combined movability of the X-ray source and thedetector on the C-arm enables the formation of x-ray images fromdifferent projection directions.

The above X-ray apparatus preferably comprises diaphragms which can beadjusted by means of actuators or motors and which define the radiationcone and hence the volume covered thereby, the adjustment of suchdiaphragms is among the imaging parameters optimized by the dataprocessing unit. The volume represented in the X-ray image can then belimited to a minimum as required for the representation, thus minimizingthe radiation load.

In conformity with a further embodiment of the imaging system, the dataprocessing unit is coupled to signal leads, for example, leads for anelectrocardiogram (ECG) and/or a respiration sensor. The calculations tobe executed by the data processing unit can be further specified bytaking into account further sensor information. For example, thechanging of the shape of the body of a patient which is associated withthe heartbeat or the respiration can be taken into account when theposition of the object is determined and associated with thethree-dimensional representation. Furthermore, there may be provided asignal lead for the connection of a localization device which serves todetermine the current position of the object. The localization devicemay be supported, for example, by a separate imaging method, by alocalization method by means of electromagnetic field measurements(“active localizer”), or in special applications also by thedetermination of the spatial configuration of an instrument carrierprojecting from the body volume.

The imaging system can notably be configured or extended in such amanner that it is capable of carrying out a method of the kind setforth.

Thus, the imaging system may be arranged, for example, to determine theposition of the object from a first two-dimensional image which has beengenerated by means of the same method as the optimized two-dimensionalimage, because in this case only a single imaging system is required.

The nature of the imaging parameters optimally determined by the imagingsystem is dependent on the imaging methods used. Examples in thisrespect have already been given above.

The feasible locations of the object can notably be vessels within abiological body volume, the data processing unit in that case preferablybeing arranged to define the optimum imaging parameters in such a mannerthat the vascular segment in which the object is situated is projectedessentially in a planar fashion in the two-dimensional image.

In conformity with a further version of the imaging system, it mayinclude a device (monitor, printer, etc.) for the reproduction of imagesand be arranged in such a manner that the two-dimensional image isdisplayed so as to be superposed on an image formed from thethree-dimensional representation with entirely or partly the sameimaging parameters, the image formed from the three-dimensionalrepresentation preferably reproducing a larger area than thetwo-dimensional image. The advantages of such a common display havealready been mentioned.

The invention will be described in detail hereinafter, by way ofexample, with reference to the Figures. Therein:

FIG. 1 shows a diagram of the imaging system in accordance with theinvention, and

FIG. 2 illustrates the X-ray projection of a body volume with a vascularsystem and a catheter introduced therein.

FIG. 1 shows an example of the application of the invention in the formof an imaging system which is used to track the movement of the tip of acatheter through the vascular system of a patient 10. In the context ofcardiological interventions, the catheter may be, for example, acatheter for a PTCA (Percutaneous Transluminal Coronary Angioplasty), aperfusion an electrophysiology (EP) mapping or an ablation.

A two-dimensional image of the body volume of interest is formed inknown manner by means of an X-ray apparatus 3 which comprises an X-raysource 7 and an X-ray detector 8 which are attached to oppositelysituated ends of a C-arm 9. The C-arm 9 can be pivoted in such a mannerthat the X-ray apparatus acquires two-dimensional images of the bodyvolume 10 of interest from different projection directions. The imagesare available as “live” (real-time) fluoroscopic images 4 during themedical intervention.

A suitably programmed data processing unit in the module 5 calculatesthe position of the tip of the catheter within the body of the patientfrom the two-dimensional images 4. To this end, the module 5 receivesinformation as regards the position of the X-ray tube 7 and the detector8 relative to the patient 10. Preferably, the module 5 also takes intoaccount signals from sensors 6, for example, an ECG or signals from arespiration sensor in order to enhance the precision of thedetermination of the position. Alternatively, the current position ofthe tip of the catheter can also be determined by means of other methodssuch as, for example, by means of ultrasound imaging or by means of anactive localizer which determines its position in space relative to amagnetic field.

The position of the tip of the catheter thus determined is subsequentlyapplied to another data processing unit or to another programming module2 within the same data processing unit, said module 2 additionallyhaving access to a stored three-dimensional representation 1 of thevascular tree within the body volume of interest. The data of thisthree-dimensional representation, vectorally and/or point-wisedescribing the course of the vessels in a three-dimensional co-ordinatesystem, has been acquired by means of a three-dimensional imaging method(for example, CT, MR, CRA, 3DUS, etc.) prior to the currentintervention. The three-dimensional representation can be acquirednotably by means of rotation angiography while utilizing the X-rayapparatus 3 which is also used during the current intervention.

The module 2 associates the (two-dimensional) position of the tip of thecatheter as provided by the module 5 with the corresponding(three-dimensional) position of the tip of the catheter within thevascular tree. Methods of associating corresponding points in differentrepresentations of the same volume in this manner are known (forexample, from U.S. Pat. No. 6,317,621 B1) and hence will not beelaborated herein. This association utilizes the fact that the cathetermoves through the vascular system and that hence its tip must besituated in the vascular tree described by the three-dimensionalrepresentation.

After the determination of the position of the tip of the catheter inthe vascular tree, the module 2 determines new imaging parameters whichhave been optimized in conformity with given optimization criteria.Optimization of this kind is obtained for the system shown in FIG. 1,that is, notably when the tip of the catheter is projected in a planarfashion, that is, from a direction extending perpendicularly to thelocal vascular segment in which the tip of the catheter is currentlysituated. In as far as there more of such directions (there aregenerally two 180° offset directions), preferably the direction ischosen which necessitates the least changes of settings of the X-rayapparatus. The planar projection of said vascular segment offers theadvantage that it reproduces this segment with a maximum length, so thatthe further advancement of the tip of the catheter can be observed withthe highest resolution.

Furthermore, the module 2 can calculate those boundaries of the X-raycone that still lead to adequate imaging of the tip of the catheter ofinterest. These boundaries can be defined, for example, in such a mannerthat the resultant two-dimensional projection has the shape of anelongate rectangle in which the tip of the catheter is situated near ashort side and the associated vascular segment, being adjacent in thedirection of propagation, extends to the oppositely situated short sideof the rectangle. Such a representation would actually be limited to theanticipated future path of motion of the catheter.

After the determination of the projection direction and the projectioncone as well as possibly further imaging properties, for example, theradiation intensity of the X-ray source 7, said variables are applied tothe X-ray apparatus 3 in which the corresponding settings are realized.This means that in particular the C-arm 9 is rotated until the X-raysource 7 and the detector 8 are situated in the predetermined projectiondirection, and that X-ray attenuating diaphragm wedges and/or X-raytransparent diaphragms are motor-driven to the position in which theimaging window determined is obtained. Subsequently, a new, optimizedX-ray image can be generated.

Not being shown in detail in FIG. 1, the three-dimensionalrepresentation 1 of the vascular system and the fluoroscopic real-timeimages 4 from the same optimum projection angle determined can bedisplayed in superposed form so as to provide the user with additionalinformation. Preferably, the projection of the three-dimensionalrepresentation 1 covers a larger area than the real-time images 4, sothat the physician can look around in a comparatively large area aroundthe object while at the same time the fluoroscopic images acquired whileexposing the object to a radiation load can be limited to an as small aspossible area.

The described imaging system and the associated imaging methodeliminates the time-consuming re-positioning of the X-ray apparatusduring complex medical interventions by utilizing an intelligentnavigation control system. The medical staff no longer has to carry outthe re-positioning of the C-arm 9, so that not in the least the X-raydose whereto the patient is exposed is reduced. This dose isadditionally reduced in that the image is automatically limited to therequired imaging window.

FIG. 2 shows the images on which the method in accordance with theinvention is based. The Fig. shows the vascular tree 14 which has beenmeasured in advance and documented in a three-dimensionalrepresentation, and also the front segment of a catheter 12 with thecatheter tip 15 inserted therein. Also shown is the X-ray cone 1 whichproduces a two-dimensional projection image 13 in the plane of the X-raydetector 8 (FIG. 1) (corresponding to the fluoroscopic images 4 of FIG.1).

After the determination of the position of the tip of the catheter 15 inthe three-dimensional vascular tree 14 by means of the module 2 of FIG.1, the projection direction produces an optimum image of the catheter 12and the tip of the catheter 15 can be determined while taking intoaccount the course of the vessels. As is shown in FIG. 2, this maynotably be a projection from a direction perpendicular to thelongitudinal direction of the catheter 12 or of the surrounding segmentof the vascular tree.

Even though the invention has been described in conjunction with thedisplacement of an instrument through the vascular system of a patient,it is by no means restricted to this application. In thebiological/medical field, for example, the motion of a natural objectthrough the body could also be observed, for example, the motion of ablood clot through the vascular system or the transport of a substanceor excitation potential along other paths such as, for example, nervetracts.

Furthermore, the invention can also be used, for example, in toolengineering applications. For example, the object could be the hand of a(multi-jointed) robot arm which is to be moved under the control offeedback signals from a video camera so as to perform a task on aspatially complex object. Using the method in accordance with theinvention, in such a case an optimum position of the video camera couldbe adjusted, notably a position which first of all offers anunobstructed view of the hand of the robot and secondly images the handwith the highest resolution, that is, for example, in a planar fashion.

1. A method of optimizing a two-dimensional image of a body volume which contains an object, in which method a) a three-dimensional representation of feasible locations of the object within the body volume is acquired; b) the current position of the object is determined and associated with the three-dimensional representation; c) imaging parameters which are optimum in respect of the position of the object are determined by means of the three-dimensional representation, and d) a two-dimensional image of the body volume is generated by means of said optimum imaging parameters.
 2. A method as claimed in claim 1, wherein the two-dimensional image is a projection of the body volume which has been generated by means of X-rays.
 3. An imaging system for forming a two-dimensional image of a body volume which contains an object, which system comprises a data processing unit with a memory which stores a three-dimensional representation of feasible locations of the object within the body volume, the data processing unit being arranged a) to determine imaging parameters which are optimum in respect of the current position of the object by means of the three-dimensional representation; b) to control the imaging system in such a manner that it generates a two-dimensional image with said imaging parameters.
 4. An imaging system as claimed in claim 3, wherein it includes an X-ray apparatus with an X-ray source and a detector which are attached to a movable C-arm.
 5. An imaging system as claimed in claim 4, wherein the X-ray apparatus includes adjustable diaphragms whose adjustment forms part of the imaging parameters optimized by the data processing unit.
 6. An imaging system as claimed in claim 3, wherein the data processing unit is coupled to signal leads, notably for an ECG, of a respiration sensor and/or of a localizing device for the object.
 7. An imaging system as claimed in claim 3, wherein it is arranged to determine the current position of the object from a two-dimensional image.
 8. An imaging system as claimed in claim 3, wherein the imaging parameters define a sectional plane, a projection direction, the position of a radiation source, the position of an imaging radiation detector, the shape of an imaging window, the position of radiation-attenuating diaphragm elements, variances in the radiation field across an irradiated surface, a radiation quality, a radiation intensity, the current and/or the voltage of a radiation source and/or an exposure time.
 9. An imaging system as claimed in claim 3, wherein the feasible locations of the object are vessels within a biological body volume, and that the data processing unit is arranged to define the optimum imaging parameters in such a manner that the segment of the vascular tree in which the object is situated is projected essentially in a planar fashion in the two-dimensional image.
 10. An imaging system as claimed in claim 3, wherein it includes a device for the formation of images and is arranged to display the two-dimensional image in superposed form together with an image formed from the three-dimensional representation with completely the same or partly the same imaging parameters, the image formed from the three-dimensional representation preferably reproducing an area which is larger than that reproduced by the two-dimensional image. 